The disclosed invention relates to the field of the optical beam manipulation in optical disk storage devices, and in particular to preventing undesirable reflections from the disk surface from interfering with the proper detection of the coarse servo tracking signal reflected from the disk surface.
In optical storage, information is stored on media which generally consists of a reflecting substrate, a phase layer, an active layer, and a protective overcoat. A protective overcoat is used because the active layer is generally rather fragile and easily damaged. In some devices, the protective overcoat also serves as a dust defocusing layer. The information is initially recorded as changes in reflectivity between the "marked" and the "unmarked" regions of the media. The unreflective region exists as a result of destructive interference between the incident beam reflected at the active layer and the beam reflected off of the substrate. When an optical hole is created in the active layer, no light is reflected at the active layer, so that the light reflected by the substrate returns through the system with out interference, causing a bright "spot" to be detected.
It is well known in the art to use a focused radiation beam, usually a laser, to record and read both digital and analog information on such a recording media. With such focused beams, the light is generally focused to as small a spot as feasible in order to store data as densely as possible on the recording surface. In rotating disk storage devices, information is stored in either concentric or spiral recording tracks. For proper device operation, the light beam must be properly focused on the disk surface. The focused beam must be properly tracked across the disk surface. There have been a number of different methods taught for properly focusing the beams on the disk surface, all requiring a tightly focused spot on the disk media.
Likewise, tracking of the device beams over the disk surface has generally followed well known techniques. For fine tracking of the focused beam over the disk surface, prior devices have taught the focusing of a pair of tracking spots on opposite edges of a data track on the disk surface, and comparing the strength of the reflected signals. Thus, tracking also requires tightly focused spots.
However, in random access devices using disk shaped carriers, it has generally been necessary to provide for the gross (coarse) translation of the optical elements over the disk surface from one area of the disk to another. The coarse translation of the beams over the disk surface has generally been accomplished by mounting some portion of the optics in a carriage actuator, and moving the carriage actuator over the disk surface to the area to be read or written. Relatively widely spaced coarse servo tracks are then used to determine actuator and optical element position as the actuator moves across the disk surface. To detect these relatively wide spaced tracks, a broad spot is generally preferred.
Because two fundamentally differnt types of spots are needed at the disk surface, there has been increased interest in the use of multi-laser systems. The first laser is used to supply the coarse servo beam. The second laser is used to supply to fine tracking focusing and data reading beam.
However, in systems having the protective overcoats, unwanted reflections from the protective overcoat can create signal detection problems. The beam reflected from the protective layer can interfere with the beam reflected from the active layer. Since the protective overcoat will vary slightly in thickness, the phase difference between the light reflected from the active layer and the light reflected from the protective overcoat will also vary as a beam is translated over the disk surface. Sometimes the beams reflected from the protective overcoat will constructively interfere with the beams reflected from the active layer, and sometimes the beams reflected from the protective layer will destructively interfere. This random variation in interference can result in serious signal detection problems.
Both the read beams and the coarse servo beams have some portion of their light reflected by the protective overcoat. However, the read beams are focused onto the disk active layer in diffraction limited spots. With a diffraction limited spot, a beam reflected from the protective overcoat will have a different radii of curvature than one reflected from the active layer, so the constructive and destructive interferences tend to cancel out at the detector, not severely effecting the read beam signal sensed by the read detectors.
However, because the coarse servo beam is focused on the disk surface as a broad spot in at least one of its optical axes, the radius of curvature of that portion of the coarse servo beam reflected from the protective layer will much more closely match that of a beam reflected from the active layer, so that the respective constructive and destructive interferences can result in unreliable coarse servo signals falling on the coarse servo detectors.
What is needed then is a means for efficiently and completely preventing light reflected from the protective overcoat from interfering with light reflected from the active layer. The present invention discloses such a means. The present invention discloses a subaperture coarse servo optical system, wherein the coarse servo beam can be efficiently reflected from the disk so that reflection from the protective overcoat does not interfere with the proper detection of the coarse servo beam.
It is an object of the disclosed invention to provide a system for delivering a coarse servo optical beam in a random access optical disk storage unit.
It is another object of the disclosed invention to provide a coarse servo optical beam which does not create undesirable interference between beams reflected from the various interfaces of the optical recording media.
It is yet another object of the disclosed invention to provide a means for preventing undesired interference between the coarse servo beams reflected from the media protective overcoat and those reflected from the active layer.